a. Technical Field
This invention relates to the field of regulated current power supplies and, in particular, to a current limiting feature therefor. The output current is sensed and regulated and, in addition, in the event of a high impedance short condition, a sampling circuit senses the condition and regulates the output current accordingly to correct for the condition.
b. Description of the Related Art
A number of techniques are known for limiting output current in a regulated power supply. Some of these include 1) setting a power limit via a controller integrated circuit and 2) depending on a voltage collapse on an auxiliary winding at short circuit on the output. Neither of these techniques involves a direct monitoring of the output current but in stead result in a power setting that translates to a current setting through well known power to current relationships in the power supply.
Early generations of power supplies for cable distribution amplifiers involved fusing to protect for an overcurrent condition. There were typically provided current fuses on the output or at the input or both with or without control circuitry to trip the fuse. Such fuses were not normally resettable and so, when they activated, maintenance personnel would be required to visit the amplifier site for replacement of the current fuses in the field. Of course, field replacement of fuses is expensive and tedious. The result was that cable television service providers would require manufacturers to provide higher and higher value fuse protection that would only protect the power supply and related equipment from dead shorts. Alternatively, they would violate recommended current ratings and intentionally install an inappropriately high current value fuse. Of course, damage to the circuitry could be the result. Consequently, these fuses would not protect the equipment from high impedance shorts and would not fire under those conditions. High value fuses protect against dead shorts but not against these high impedance current faults. Thus, the present problem solved by the present invention arose over time. Also, monitoring power at the output rather than current can allow a potentially damaging current to flow on the occurrence of a high impedance short circuit condition before the preset power limit cuts in and a controller can interrupt the current flow.
Segger, U.S. Pat. No. 4,736,264, discloses a primary switched-mode power supply including an opto-isolator coupled to the secondary side of a transformer. Segger's simple over-current protection does not correct the problem solved by the present invention.
Consequently, there remains a need in the art to provide a solution to the potentially damaging high impedance over-current problem of prior art power supplies, especially those used in cable distribution systems.